Ultrasonic driving element

ABSTRACT

An an ultrasonic drive element having a plurality of layers of piezoelectric material, arranged one on top of another to form a stack, between each of which is located a contacting layer for application of an operating voltage. Each of the contacting layers is subdivided into at least two electrically unconnected subregions, at least one of which has an electrode for making contact to the adjacent piezoelectric layers. By applying suitable alternating-current voltages to the electrode, the surface of the layer stack is excited into mechanical oscillations which contain a motion component parallel to the plane of said surface.

FIELD OF THE INVENTION

The present invention relates to an ultrasonic drive element.

BACKGROUND INFORMATION

The piezoelectric effect has been used industrially for some time. Inorder to increase the displacement travel, elements constructed with themultilayer technique are often offered, for example by the PHYSIKINSTRUMENTE company in Catalog 107 of 1990. They consist of a pluralityof thin piezoelectric layers, placed one on top of another, with athickness of, for example, 20, to 500 um, between each of which anelectrode layer is applied for electrical contacting. In a firstembodiment having a large number of layers arranged one on top ofanother, such elements generate a linear motion by application of adirect-current voltage, and a linear oscillation by application of analternating-current voltage. A second embodiment having in each case twolaterally placed layers utilizes the secondary piezoelectric effect andyields a flexural motion when a direct-current voltage is applied, and aflexural oscillation when an alternating-current voltage is applied. Adisplacement effect going beyond the maximum linear stroke or maximumflexion cannot be achieved with these elements.

A rotatory ultrasonic drive having an unlimited displacement range hasbeen disclosed by the SHINSEI company. It is based on an annularpiezoceramic which is divided into two separately activatable excitationregions, each having 8 segments. By applying two alternating-currentvoltages, offset 90° from one another in time, to the two excitationregions, a flexural traveling wave is generated in a stator adhesivelybonded to the piezo ring. This can be used, by way of a friction layer,to drive a rotor. Other improvements to traveling-wave motors operatingon the principle of the SHINSEI motor have been disclosed.

The traveling-wave principle is well-suited for rotatory motions, but itis unsuitable for linear drives. Attempts to implement a linearultrasonic traveling wave motor by cutting open the piezo ring used inthe SHINSEI motor have failed. The reasons for this are, in particular,the reflections which occur at the ends of the cut-open piezo structure,which disrupt the energy flux transported with the traveling wave andthus the entire motion. Many attempts to prevent the reflections andgenerate a traveling wave in a finite piezoelectric beam have so faryielded no industrially exploitable solutions.

Disclosed solutions for carrying out a linear drive mechanism are basedon the use of discrete individual actuators which are excited by theapplication of suitable voltages in such a way that an elliptical motionfor a surface point results. According to a solution described inEuropean Patent Application No. 297 574, two mutually perpendicularlongitudinal oscillations are used for this purpose; U.S. Pat. No.4,763,776, provides two mutually perpendicular flexural oscillations.

It is an object of the present invention to provide an easily handledultrasonic motor which is suitable for linear displacement motions.

SUMMARY OF THE INVENTION

This object is achieved by providing an ultrasonic drive element. Theultrasonic drive element according to the present invention offers anadvantage that it can be manufactured easily and with a great variety offorms. Driving can occur in such a way that the surface points which arepressed against the element being driven execute elliptical motions; andalso using a plunger mechanism whereby the surface points execute aplunger motion against the part being driven. In both cases, it is easyto reverse the motion. The ultrasonic drive element according to thepresent invention provides additional advantages based on the principlesof the ultrasonic driver element. In particular, driving can take placedirectly, i.e. without a gear train. High forces, on the order of 10² N,are possible, along with low velocities on the order of 10 mm/s. Theinstallation space required is small by comparison with the poweroutput.

One of the underlying principles for the drive element according to thepresent invention is to configure the electrical contacting of thepiezoelectric layers in such a way that specific excitation of certainmechanical oscillations is possible. In a first embodiment, thecontacting layers are each subdivided into subregions, in each of whichan electrode is configured which can be activated separately. Byapplying alternating-current voltages that have been suitablyphase-shifted with respect to one another to the electrodes, thepiezoelectric layers can be selectably excited into longitudinal,flexural, or elliptical oscillations.

An alternative possibility for exciting longitudinal or flexuraloscillations in adjacently located regions of a piezoelectric layerprovides, for example, for longitudinal oscillations to be excited inthe upper region of a drive element, and flexural oscillations in thelower region. A further alternative provides, for example, forlongitudinal oscillations to be excited in the inner regions of a driveelement, and flexural oscillations in the outer region of a driveelement.

To implement a particularly effective drive, an excitation of thelongitudinal/flexural oscillations can occur in resonant fashion. Inorder to suppress undesired energy exchange between the two oscillationforms, the drive element is advantageously dimensioned so that thenatural frequencies of the two oscillation forms are far apart.Advantageously, the dimensioning is such that the ratio of the naturalfrequencies to one another is integral. Another advantageous way ofmaking possible simultaneous resonant excitation of longitudinal andflexural oscillations is to excite the flexural oscillations in thesecond flexural mode. The result is not only the capability forsimultaneous resonant excitation of both oscillation forms, but also theadvantage of reduced noise production.

It is advantageous to embody drive elements in the form of multiplepiezoelectric layers stacked into turrets, which are arranged increnellated fashion next to one another, and are activated in mutuallytuned fashion. Especially large forces can be applied in this manner. Inaddition, with the use of the turret concept it is easy to implementrotatory drives as well, by arranging the turrets in annular fashion.Another embodiment according to the present invention provides forturrets arranged one next to another, which additionally are joined byturret-like bridge stacks which are of lesser height. In thisembodiment, the turrets and bridge stacks are each excited only tolongitudinal oscillations.

In yet another embodiment according to the present invention, only aportion of each piezoelectric layer is electrically contacted in eachcase, while the remaining portion is not electrically contacted. Withthe resulting asymmetrical contacting, both flexural and linearoscillations can easily be excited by means of only a singlealternating-current voltage.

An advantageous variation of this embodiment provides for each layer tobe subdivided into three separately contacted subregions, the two outerones being activated so that they oscillate in opposite directions. Thecenter region is activated in phase with one or the other outer regiondepending on the desired direction of motion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the construction of a multilayer drive element according tothe present invention.

FIG. 2a shows a first embodiment of a drive element constructed frommultiple turrets.

FIG. 2b shows a second embodiment of the drive element constructed fromthe multiple turrets.

FIG. 2c shows a third embodiment of the drive element constructed fromthe multiple turrets.

FIG. 2d shows a fourth embodiment of the drive element constructed fromthe multiple turrets.

FIG. 3a shows a first division of excitation regions.

FIG. 3b shows a second division of the excitation regions.

FIG. 4a shows a first form of the drive element constructed frommultiple turrets.

FIG. 4b shows a second form of the drive element constructed frommultiple turrets.

FIG. 4c shows a third form of the drive element constructed frommultiple turrets.

FIG. 4d shows a fourth form of the drive element constructed frommultiple turrets.

FIG. 4e shows a fifth form of the drive element constructed frommultiple turrets.

FIG. 4f shows a sixth form of the drive element constructed frommultiple turrets.

FIG. 5a shows a first position of the drive element in a sequence thatis constructed from the turrets and bridge stacks.

FIG. 5b shows a second position of the drive element in a sequence thatis constructed from the turrets and bridge stacks.

FIG. 5c shows a third position of the drive element in a sequence thatis constructed from the turrets and bridge stacks.

FIG. 5d shows a fourth position of the drive element in a sequence thatis constructed from the turrets and bridge stacks.

FIG. 6 shows another embodiment of the drive element constructed fromthe turrets and bridge stacks.

FIG. 7a shows a side view of the drive element configured in a V-shape.

FIG. 7b shows a prospective view of the drive element illustrated inFIG. 7a.

FIG. 8a shows a side view of the drive element excitable in a singlemanner, along with oscillation patterns.

FIG. 8b shows a top view of a subregion of the drive element illustratedin FIG. 8a.

FIG. 8c shows a first movement position of the oscillations relating tothe drive element illustrated in FIG. 8a.

FIG. 8d shows a second movement position of the oscillations relating tothe drive element illustrated in FIG. 8a.

FIG. 9 shows yet another embodiment of the drive element excitable insingle-phase fashion according to the present invention having threeelectrodes per layer.

FIG. 10a shows a first configuration of the drive element according tothe present invention.

FIG. 10b shows a second configuration of the drive element according tothe present invention.

FIG. 10c shows a third configuration of the drive element according tothe present invention.

FIG. 10d shows a fourth configuration of the drive element according tothe present invention.

FIG. 11 shows an arrangement for driving an object.

FIG. 12 shows the drive element suitable for exciting the secondflexural mode according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the structural principle of a drive element accordingto the present invention. It is created by alternately layering on topof one another layers 10 of piezoelectric material, hereinafter calledpiezo layers, and contacting layers 12 for electrical contacting of thepiezoelectric layers 10. Contacting layers 12 are each subdivided intoat least two electrically separate regions. They are composed of regionsof electrically conductive electrode material, and regions ofnonconductive filler material. The regions of an electrically conductiveelectrode material define electrodes 13, 14, 15 for excitation ofpiezoelectric layer 10. The filler material is applied to points 16where no electrode is intended to be located, and serves in particularto subdivide contacting layers 12. Via electrical supply leads (notdepicted), a first voltage U₁ is applied to electrodes 13 configured incontacting layers 12, a second voltage U₂ to electrodes 14, and groundpotential U_(G) to electrodes 15.

A layered arrangement 10 to 16 as shown in FIG. 1 can be manufactured asfollows: A suitable base material, for example lead zirconate titanate(Pb(Zr_(x) Ti_(1-x))O₃, where x=0.4 to 0.6), or alternatively a materialderiving from the complex ternary systems (such as perhaps PbZiO₃--Pb(Me_(1/3) Nb_(2/3))O₃ where Me=Mg, Mn, Co, such that the Pb can bepartially substituted by Ba or Sr), is first processed into a finepowder. This is mixed with an organic binder material, plasticizers, andaqueous or organic solvents into a dispersion, called a "slurry." Theslurry is then cast by means of a casting apparatus to yield a film; thesolvent is evaporated off in the drying duct adjacent thereto. Theresult is a film that is in particular characterized by greatflexibility, called the "green film." Planar electrodes are now appliedonto the green film, preferably by silk-screening. The desired stackedstructure, with internally located electrodes, is obtained by subsequentstacking, pressing, and cutting of the films. The binder is burned outof the resulting green multilayer, and sintering is then performed inthe same firing. Lastly, the completed piezoceramic element can bepolarized by means of the internally located electrodes.

The drive element constructed as shown in FIG. 1 is, according to afirst embodiment according to the present invention, excited innonresonant, two-phase fashion. Thus, alternating-current voltages U₁,U₂, shifted 90° in phase from one another and having, for example, thefollowing form:

U₁ =U₀ *sin (ωt) and U₂ =U₀ *cos (ωt)

or, written differently:

U₁ =U₀ *sin (ωt-45°) and U₂ =U₀ *sin (ωt+45°), where U₀ is amplitude,

are applied to the electrode pairs 13/15 and 14/15, respectively.

The excitation is typically performed at high frequency in theultrasonic range, with frequencies of, for example, f=ω/2π>20,000 Hz,which is nonresonant, i.e. the frequency of the electrical excitation isnot equal to the mechanical natural frequencies of the drive element.The latter are generally higher than the excitation frequency;excitation is noncritical. In this case, the drive element tracks theexcitation frequency directly, with no phase distortion. At least twodifferent oscillations are excited in the drive element by the twovoltages U₁, U₂. These voltages are superimposed in the drive element,with the result that points on the surface of the drive element executean elliptical motion. The traverse direction of the ellipse depends onwhich of the voltages U₁ or U₂ leads the other by 90°. Reversal of themotion is achieved by rotating one of the voltages.

FIG. 2a shows a first embodiment of a drive element according to thepresent invention constructed using the multilayer technique. The driveelement includes a plurality of stacks of piezoelectric and contactinglayers, embodied in the form of turrets 20. All the turrets 20 areinterconnected by means of a base 31 which contains at least one (andpossibly more than one) continuous piezoelectric layers 30. A pluralityof turrets 20, arranged in crenellated fashion next to one another alongan extension direction R, constitute in each case a drive element in theform of a multipiezo actuator.

As illustrated in FIG. 2b, contacting layers 12 arranged betweenpiezoelectric layers 10 are each subdivided into two regions 101, 102,each of which constitutes a separately activatable electrode. The tworegions 101, 102 will be referred to hereinafter as left region 101 andright region 102, respectively. A voltage U₁ is applied to therespective electrodes of a turret 22 configured in the left regions 101of contacting layers 12, and a voltage U₂ to the right electrodes 102.If, for example, piezoelectric layers 10 are polarized in the samedirection, left electrodes 101 and right electrodes 102 are each excitedwith identical voltages with no phase shift, turrets 22 execute alongitudinal oscillation. If, on the other hand, electrodes 101 and 102are excited with voltages that are identical but shifted 180° out ofphase with one another--for example U₁ =-sin (ωt), and U₂ =sin (ωt)--theresult is a flexural oscillation. In order to excite both oscillationforms simultaneously, voltages shifted 90° out of phase, for example

    U.sub.1 =sin (ωt-45°)

    U.sub.2 =sin (ωt+45°)

are applied to left electrodes 101 and right electrodes 102,respectively.

The longitudinal oscillations H and flexural oscillations B that arethereby excited simultaneously are superimposed on one another, andcause surface 21 of a turret 22 to execute an elliptical motion with acomponent H perpendicular to surface 21 of turret 22 and a component Bparallel to extension direction R of the turret row. The resultingelliptical motion for a point P on the surface of turret 20 is indicatedabove the left turret in FIG. 2b. The component therein parallel tosurface 21 of turret 22 can, in known fashion, be used to drive anobject that is moved. In accordance with the indicated position of theellipse, direction X of the resulting motion of an object being driventhen extends parallel to extension direction R of the turret row, asindicated by the arrow on the right turret.

Because of the oscillation excitation occurring separately in eachturret 22, a single turret already executes a linear motion. The use ofa turret row as indicated in FIG. 2a is a convenient way to establish adesired force; moreover, this improves the linearity of the motion.

The turret concept depicted in FIG. 2 allows for a number ofadvantageous variants. For example, contacting layers 12 can be dividedinto regions not just perpendicular to extension direction R to producea left/right division, but also, as shown in FIG. 2c, parallel todirection R to produce a front/rear division. As a result, as indicatedby the arrow on left turret 24, output drive direction Y is also rotated90° as compared with what is obtained with a left/right arrangement.Direction Y is then perpendicular to extension direction R of the turretrow.

It may also be advantageous to divide each contacting layer 12 into fourregions--right front 105, right rear 106, left front 103, left rear104--as indicated in FIG. 2d. In this fashion, motions can be generatedboth parallel to extension direction R of a row of turrets 26, and alsoperpendicular thereto. With an appropriate activation, the basic outputdrive directions X, Y can be superimposed to yield an overall motion inany desired direction in the plane parallel to the turret surface.

The variety of possible motions can be further expanded by arrangingturrets in fields. If, for example, 10×10 individually activatableturrets are arranged in such a field, an object driven above it can bemoved in two translational spatial directions. In addition, a rotationabout the axis orthogonal to the translational directions can besuperimposed on the translational movement.

According to another embodiment of the present invention, thelongitudinal and flexural oscillations are excited not in one layer, butin spatially separated layers. Examples for this variant are shown inFIGS. 3a and 3b. In the arrangement shown in FIG. 3a, voltages cos(ωt)and -cos(ωt), shifted 180° in phase from one another, are applied to thelower regions 32, 33 of the turret; and voltages sin(ωt) of identicalphase, but shifted 90° in phase with respect to those applied to thelower regions, 32, 33 are applied to upper regions 34, 35. As a result,flexural oscillations as indicated by double arrows in oppositedirections are excited in the lower part of the turret, and longitudinaloscillations, indicated by double arrows in the same direction, areexcited in the upper region. In the embodiment shown in FIG. 3b, centerregion 37 executes longitudinal oscillations due to application of avoltage sin(ωt), while flexural oscillations are excited between outerregions 36, 38 by the application of voltages cos(ωt) and -cos(ωt) thatare shifted 180° in phase from one another.

A number of equivalent possibilities exist with regard to activating adrive element constructed with multiple turrets. In addition toactivation of all the turrets in exactly the same direction, as depictedin FIG. 4a, they can also, as indicated in FIG. 4b, be activated in sucha way that neighboring turrets 41, 42 behave oppositely to one another.While one half of the turrets 41 move, for example, to the right andupward, the other half of the turrets 42, the turrets locatedrespectively between the turrets 41 move, are traveling back down and tothe left. Activation of this kind can be implemented by increasing thenumber of excitation supply leads from two to four (the number of groundlines, not given further consideration here, is to be increasedanalogously) or, more favorably, by a suitable configuration of theelectrodes and/or polarization of the piezo layers, only two supplyleads then being required in unchanged fashion.

A further activation is possible, as shown in FIG. 4c, by activating theturrets so that their surfaces simulate a positively excited travelingwave.

It is furthermore possible to implement rotatory drives using stacks ofpiezoelectric layers and contacting layers embodied in the form ofturrets. FIG. 4d shows a turret row embodied in the form of a ring, inwhich surfaces 21 are aligned on the inside of a cylinder. Thearrangement allows, in particular, rotary motions about a concentricallyarranged axis 43.

FIG. 4e shows an arrangement with turret surfaces 21 arranged on theoutside of a cylinder. The arrangement allows, in particular, motionsrelative to the inner surface of a surrounding cylinder 44.

FIG. 4f shows, as a further possible embodiment, of the presentinvention an annular arrangement in which surfaces 21 of the turrets arearranged radially in one plane. With such an arrangement, motions of anykind can be implemented relative to the plane lying through surfaces 21of the turrets.

A further embodiment of the drive element according to the presentinvention based on the use of turrets is shown in FIGS. 5a-5d, in whichturrets 50 arranged next to one another are joined by bridge stacks 51.Bridge stacks 51 are constructed like turrets, but have a lower stackheight. They are advantageously arranged between turrets 50 in such away that they touch (contact) neither the element being driven nor base59 on which turrets 50 are mounted. The electrodes of bridge stack 51are located perpendicular to the desired elongation direction, and thesecondary piezoelectric effect of transverse contraction is used; thepolarization and electric fields are perpendicular to the desiredlongitudinal direction. Both turrets 50 and bridge stacks 51 arerespectively excited into longitudinal oscillations, the oscillationsexcited in the bridge stacks 51 lying orthogonally to those excited inturrets 50. The motions of turrets 50 and bridge stacks 51 are onceagain superimposed at surfaces 21 of turrets 50 in such a way thatbridge stacks 51 execute elliptical motions. Four stages of a motion ofthis kind are depicted in FIGS. 5a to 5d.

In the motion stage shown in FIG. 5a, left turret 50 has contracted,right turret 50 has elongated, and both are at their respective reversalpoints. At this point in time bridge stacks 51 are experiencing a zerotransition, and are neither stretched nor elongated. In the secondmotion stage as shown in FIG. 5b, turrets 50 are experiencing a zerotransition, i.e. are neither stretched nor elongated. Center bridgestack 51 is at the reversal point after a contraction, and the outerbridge stacks are at their respective reversal points after anelongation.

The third stage as shown in FIG. 5c corresponds to the first as shown inFIG. 5a, but the motion states of the turrets are interchanged. The leftturret has elongated, and the right turret contracted. The fourth stageas shown in FIG. 5d corresponds analogously to the second, the motionstates of bridge stacks 51 now being interchanged. The center one is atthe reversal point after a rotation, and each of the outer bridge stacksis at the reversal point after a contraction.

As illustrated in FIGS. 5a-5d, points P₁ and P₂ located on surfaces 21of the turrets each describe elliptical motion paths in this motionsequence. The embodiment depicted in FIGS. 5a-5d is particularly easy tomanufacture if turrets 50 and bridge stacks 51 originally terminateflush with surfaces 21, 53, i.e. are produced with a smooth surface, anda friction layer which is in contact with an element being driven isthen applied onto surfaces 21 of turrets 50.

FIG. 6 shows a further embodiment of a turret/bridge stack structureaccording to the present invention. Here each turret 50 is joined to aneighboring turret by a bridge stack 51, yielding an H-shaped structure.There is no connection to the respective other neighboring turrets 50.

According to a further embodiment depicted in FIGS. 7a and 7b, the driveelement has a V-shaped structure. Electrodes 73, 73 are configured inthe contacting layers in the respective arms of the structure. In theexample depicted, the angle enclosed by the arms is approximately 90°.For use, this V-shaped element is arranged so that electrodes 73, 74 areeach inclined 45° with respect to the motion direction X, indicated byan arrow, of an element being driven. Drive contact takes place via afriction surface 71 configured at the end face of the V-shapedstructure. The secondary piezoelectric effect is used. Electrodes 73, 74in the arms are excited in such a way that they execute oscillationsthat are shifted 90° in phase from one another. The result of this atfriction surface 71 is an elliptical motion sequence, as illustrated inFIG. 7a with reference to four motion stages for a point P₃ on frictionsurface 71.

Excitation of the embodiments of a drive element described above is notlimited to frequencies in the subcritical or supercritical range, i.e.well below or above the natural frequencies of the excited oscillationforms, but can also take place in resonant mode, at or near the naturalfrequency of one of the participating oscillation forms. A consequenceof such resonant excitation is that the resonantly oscillatingoscillation form lags the excitation by 90°. This can be compensated forby correspondingly shifting the pertinent first alternating-currentvoltage by 90° with respect to the second alternating-current voltage.The advantageous result of the shift is that both oscillation forms canbe excited by an excitation having the same phase position. A resonantexcitation of this kind is, however, difficult to tune and comparativelysubject to disruption.

A further possibility for activating a proposed drive element includesperforming the excitation at a frequency which lies between the naturalfrequencies of the relevant oscillation forms. Here one oscillation form(ideally) lags behind the excitation with a phase shift of 180°, whilethe other oscillation form (ideally) tracks the excitation with no phaseoffset. The motion of the ellipse at the surface is thus retained, butits circulation direction is reversed. An intercritical excitation ofthis kind thus offers an advantageous way of reversing the motiondirection.

It is advantageous to activate the drive element in such a way that bothoscillation forms are resonantly excited simultaneously. This ispossible if the natural frequencies of the relevant oscillation modesinvolved are identical, which is particularly the case with the V-shapedepicted in FIGS. 7a and 7b. There is ideally a phase offset of 90° forboth oscillation forms, and the elliptical motion of the surface isretained. This activation variant is very attractive because of theresonance-related amplitude increase. Once again, however, an excitationfrequency of this kind is difficult to tune. The formation of beatsbetween the excited oscillations must, in particular, be avoided. Thisresults in an energy exchange between the oscillations in the form of asuperimposed beat oscillation, and the two oscillations involved areamplitude-modulated in phase-shifted fashion in accordance with itsfrequency. As a result, the elliptical motion of the surface of theturret periodically degenerates into a translational motion with nocomponent in the desired drive direction.

One way of configuring a drive element so as to make possible asimultaneous resonant excitation of all the oscillation formsparticipating in generation of the surface motion is by dimensioning theturrets geometrically in such a way that the natural frequencies of theparticipating oscillation forms are sufficiently different from oneanother. Advantageously, the geometrical configuration is such that thefrequencies required for oscillation excitation are related in anintegral ratio, for example 1:2. The motion form which results for thesurface points of a turret is then a figure-eight. As in the case of theelliptical motion, the component contained in the "upper part" of thismotion parallel to the surface of the turret is used to generate a drivemoment. The excitation voltages can be applied to the same electrodes,and are superimposed there. Reversal of the motion can be achieved bycontrolling the phase difference.

Another way of making possible simultaneous resonant excitation ofseveral oscillations needed to generate a surface motion is indicated inFIG. 12. Here the flexural oscillation excitation occurs in such a waythat the flexural mode excited is not the first, but the second oroptionally higher one. Although the higher flexural modes have a reducedamplitude as compared with the first, their natural frequencies arehigher. The second or higher flexural oscillation mode, and alongitudinal oscillation, can thus be excited simultaneously in resonantfashion, thus balancing out the disadvantage of the lower amplitude ofthe second flexural mode. For this purpose the turret is, as shown inFIG. 12, divided into at least four regions 121 to 124, which are thenactivated in such a way that the second flexural mode is excited withrespect to the flexural oscillation. With four regions 121 to 124 asshown in FIG. 12, this is achieved by exciting diagonally oppositeregions 121, 124 with, for example, a voltage U₃ =+A·cos (ωt), and theother two diagonally opposite regions 122, 123 with U₄ =-A·cos (ωt),where A is the amplitude of the excitation voltage. The respectiveexcitation voltage for exciting the longitudinal oscillation isadditionally superimposed on this excitation voltage.

The higher natural frequency of the second flexural oscillation also hasthe advantage that it lies well above the human hearing threshold, andmoreover well above the hearing threshold of many animals. The result isthat the noise generated by the drive is considerably reduced for all.

FIGS. 8a and 8b show a further embodiment of the drive elementillustrated in FIG. 1. The drive element includes again of a pluralityof turrets 80, arranged next to one another in crenellated fashion,which are connected by means of a common base 81 having at least onecontinuous piezoelectric layer 30. Of the piezoelectric layers 10indicated by horizontal lines, however, only a subregion 801 in eachcase is contacted by means of an electrode arrangement. The other region802 in each case is not electrically contacted. Layers 10 are dividedinto regions 801, 802 asymmetrically, as depicted in FIG. 7b withreference to a cross section through a turret along a piezoelectriclayer 10. In the example, electrically contacted region 801 is largerthan uncontacted region 802, but it can also be smaller. The advantageof the arrangement as shown in FIGS. 8a and 8b is that it can beactivated by means of only a single control voltage. Because of theasymmetry caused by the division into unequal regions, two differentoscillation forms are excited even if only one alternating-currentvoltage is applied. Turrets 80 once again each execute both a linearmotion H and a flexural motion B. The configuration of the overallmotion thus resulting by superimposition at surface 83 depends on thefrequency of the alternating-current voltage used for excitation. Ifexcitation occurs at or near the resonant frequency of the flexuraloscillation, and if the latter is (as is always assumed hereinafter)lower than the resonant frequency of the longitudinal oscillation, theflexural motion lags 90° behind the excitation. With this assumption,the longitudinal oscillation is still subcritical; it does not lagbehind the excitation (or does so only slightly). The result, althoughexcitation is occurring with only one alternating-current voltage, isonce again both a longitudinal oscillation H and a flexural oscillationB shifted 90° in phase from it. Surface 83 of a turret 80 thus executesan elliptical motion, usable for a drive, in a plane perpendicular tothe dividing line between regions 801 and 802. Because of the resonantor at least approximately resonant excitation of the flexuraloscillation, its amplitude is exaggerated. The motion executed bysurface 21 therefore corresponds to a flattened ellipse. The manner inwhich it occurs is illustrated, above the center turret 80 in FIG. 8a,with reference to four motion stages of a surface point P₄. Theinfluence of linear oscillation H on the resulting motion path is shownto the left of the ellipse, and the influence of flexural oscillation Babove the ellipse.

If the frequency of the excitation voltage is set at or near theresonant frequency for the longitudinal oscillation, surface 21 of aturret 70 moves along an ellipse that is elongated perpendicular tosurface 73. Flexural oscillation B then (theoretically) lags 180° behindthe excitation, and longitudinal oscillation H lags by 90°. Theresulting motion path of a surface point P₅ and the way in which itoccurs by the interaction of linear oscillation H and flexuraloscillation B is depicted above the right turret, once again in the formof four motion stages.

If the excitation occurs for both oscillation forms eithersubcritically, i.e. at a frequency which is much lower than the resonantfrequencies of the two oscillation forms, or supercritically, i.e. at afrequency which is much greater than the resonant frequencies of theoscillation forms, surface 21 of turret 80 executes an approximatelylinear motion which can extend, for example, from bottom left to topright. FIG. 8c shows, in the form of a motion stage diagram, how such amotion occurs in response to a linear oscillation H and a flexuraloscillation B. A motion in the opposite direction, i.e. for example fromtop left to bottom right, is achieved by applying the excitation at afrequency which lies between the natural frequencies of the flexuraloscillation and longitudinal oscillation. The manner in which thisoccurs is illustrated in FIG. 8d, which shows the motion of a pointunder the influence of linear oscillation H and flexural oscillation B,and of the resultant.

A variety of drive directions and drive mechanisms can be achieved withan arrangement as shown in FIGS. 8a and 8b by appropriate selection ofthe frequency of the excitation voltage. For example, a drive operatingon the plunger principle with a reversible motion direction can beimplemented by correspondingly alternating excitation at a frequency inthe subcritical range and at a frequency in the intercritical range.Selecting a first excitation frequency at or near the natural frequencyof one of the occurring oscillation modes, and a second frequency in theintercritical range, yields a drive which has an elliptical actuatormotion in one motion direction, and operates on the plunger principle inthe other direction.

The individual turrets 70 of a drive element can be activated in thesame fashion as discussed with reference to FIG. 4 for turrets excitedin two-phase nonresonant fashion.

FIG. 9 shows a further embodiment of a drive element according to thepresent invention that can be controlled in single-phase fashion. Herelayers 10 are each divided into three regions 901, 902, 903. Activationis carried out in such a way that the two outer regions 801, 803 are ineach case activated in opposite directions, i.e. with U_(L) =+U andU_(R) =-U, where U=U₀ sin (ωt). Depending on the desired resultingmotion direction, center region 902 is activated in the same way as oneof the outer regions 901, 903, i.e. with U_(M) =U_(R) for a drive outputto the left, or with U_(M) =U_(L) for drive output to the right. Thiscan be effected by a simple reversal in the electronics. Once again,therefore, a single excitation voltage is sufficient. Assuming that thenatural frequency of the flexural oscillation is lower than that of thelongitudinal oscillation, excitation preferably occurs at or near thenatural frequency of the flexural oscillation. An elliptical motion ofsurface 21 is produced, in the same way as described with reference toFIG. 8a. The circulation direction of the resulting motion ellipse canbe reversed by changing the activation of center part 902.

The shape of the drive elements can moreover be freely selected withinwide limits. FIGS. 10a and 10b show examples of turrets with a freelyconfigured geometry of layers 10, and FIGS. 10c and 10d show examples ofturrets with a layer cross section which varies with height.

The mounting of the drive element, and the principle by which power istransferred to the part being driven, are not described here. All knownmethods are, in principle, possible. One example of a solution for adrive using two drive elements 1001, 1002 in the form of turret rows isshown in FIG. 11. Part 112 being driven is located between driveelements 111, 113, which drive it from two sides. In order reliably tomaintain sufficient contact pressure, the arrangement shown in FIG. 11can be pressed by a resilient clamp against part 112 being driven. Inthis context, arrangement 111, 113 can be mounted in floating fashion onpart 112 being driven.

What is claimed is:
 1. An ultrasonic drive element comprising:aplurality of piezoelectric layers, a first piezoelectric layer of theplurality of piezoelectric layers arranged on top of a secondpiezoelectric layer of the plurality of piezoelectric layers to form alayer stack; and a contacting layer including at least two subregionsand being positioned between the first piezoelectric layer and thesecond piezoelectric layer, each subregion including at least oneelectrode, the electrode contacting the first and second piezoelectriclayers and receiving a respective operating alternating-current voltagethat excites a surface of the layer stack to generate mechanicaloscillations, each of the mechanical oscillations having a respectivemotion component parallel to a plane of the surface of the layer stack,wherein the respective alternating-current voltage includes a firstvoltage for exciting the layer stack to generate longitudinaloscillations, the first voltage being applied to at least a firstportion of the at least one electrode, wherein the respectivealternating-current voltage includes a second voltage, wherein thesecond voltage is shifted 90 degrees with respect to the first voltage,wherein the plurality of piezoelectric layers are arranged inturret-like stacks, a first stack of the turret-like stacks beingcrenellate-positioned next to a second stack of the turret-like stackson at least one common piezoelectric base layer and being activated sothat the first stack is polarized in a same direction as the secondstack.
 2. An ultrasonic drive element comprising:a plurality ofpiezoelectric layers, a first piezoelectric layer of the plurality ofpiezoelectric layers situated on top of a second piezoelectric layer ofthe plurality of piezoelectric layers to form a layer stack; and acontacting layer including at least two subregions and being situatedbetween the first piezoelectric layer and the second piezoelectriclayer, each of the subregions including at least one electrode, the atleast one electrode contacting the first and second piezoelectric layersand receiving a respective operating alternating-current voltage thatexcites a surface of the layer stack to generate mechanicaloscillations, wherein the respective alternating-current voltageincludes a first voltage for exciting the layer stack to generatelongitudinal oscillations, the first voltage being applied to at least afirst portion of the at least one electrode, wherein the respectivealternating-current voltage includes a second voltage, and wherein theplurality of piezoelectric layers are situated in turret-like stacks, afirst stack of the turret-like stacks being crenellate-positioned nextto a second stack of the turret-like stacks on at least one commonpiezoelectric base layer and being activated so that a polarization ofthe first stack is tuned to a polarization of the second stack.
 3. Theultrasonic drive element according to claim 2, wherein each of themechanical oscillations of the layer stack has a respective motioncomponent parallel to a plane of a surface of the layer stack.
 4. Theultrasonic drive element according to claim 2, wherein the secondvoltage is shifted 90 degrees with respect to the first voltage.
 5. Theultrasonic drive element according to claim 2, wherein the secondvoltage is shifted 180 degrees with respect to the first voltage.
 6. Theultrasonic drive element according to claim 2, wherein the first andsecond stacks are activated so that the first stack is polarized in asame direction as the second stack.
 7. The ultrasonic drive elementaccording to claim 2, wherein the turret-like stacks are annularlyarranged.
 8. The ultrasonic drive element according to claim 2, whereinall of the turret-like stacks are activated in a same manner.
 9. Theultrasonic drive element according to claim 2, wherein the turret-likestacks are activated so that adjacent turrets move in appositivedirections.
 10. The ultrasonic drive element according to claim 2,wherein the first stack is coupled to the second stack via a bridgestack.
 11. The ultrasonic drive element according to claim 10, whereinthe turret-like stacks and the bridge stack are excited to providemutually orthogonal longitudinal oscillations.
 12. The ultrasonic driveelement according to claim 2, wherein the respective alternating-currentvoltage includes a frequency substantially corresponding to a resonantfrequency for the longitudinal oscillations, the respectivealternating-current voltage being applied to the at least one electrode.13. The ultrasonic drive element according to claim 2, wherein the atleast two subregions include three adjacent subregions, respectivealternating-current voltages having opposite polarity being applied toouter portions of the three adjacent subregions, a respectivealternating-current voltage having a predetermined polarity thatgenerates a desired movement direction being applied to a center regionof the three adjacent subregions.
 14. The ultrasonic drive elementaccording to claim 2, wherein the layer stack has predetermineddimensions and the first voltage and the second voltage are of differentfrequencies to simultaneously excite bending oscillations andlongitudinal oscillations.
 15. The ultrasonic drive element according toclaim 2, wherein the electrode of the first subregion contacts thesecond subregion, the second subregion being electrically insulated,wherein the first subregion has a different site than the secondsubregion, the electrode being activated using alternating-currentvoltages which simultaneously excite a linear motion and a flexuralmotion in the layer stack.
 16. The ultrasonic drive element according toclaim 15, wherein the electrode of the first subregion contacts a firstarea of the second subregion, the first area being separate from aninsulated area of the second subregion.